The invention relates to a process for fabrication of crankshafts for piston engines from formed pieces, comprising respectively a crank web and a portion of the journal. Crankshafts for smaller piston engines especially auto and diesel engines are often produced in one piece by either being cast as ingots or drop forged. The bearing journals are subsequently machined and finally hardened. Because this work has to be carried out on a workpiece which is otherwise finished, it is relatively expensive. Crankshafts for larger engines, on the other hand, are assembled from individual parts such as are available taking into account production technology facilities. Typical solutions are given in DE-B-16 25 579 where the joint between the two parts extends through the webs and in DE-B-20 23 364 where the journals are divided. The individual parts are always connected by welding, even in those cases where the crankshaft is to be assembled according to DE-C-498 170 from pressed plate metal parts positioned side by side. The person skilled in the art also knows from DE-C-848 728 that in order to save weight, the crankshaft may be hollow and that the journals and webs may be made from different materials, each of which is better suited for accommodating the different specific loads. From DE-A-19 22 488 an intermediate layer is known which serves to improve the adhesion between the individual parts connected by electron beam welding. DE-A-19 51 097 shows that before the individual parts are assembled, a finished driving element (in this case a gear) may be slid onto one of them and then fixed. In the same publication a person skilled in the art will find the proposal to design the individual parts at the separation faces in such a way that prior to welding they may be inserted into each other to provide an assembly aid.
For connecting the crankshaft to the pistons it is common to use divided connecting rods which are also relatively expensive to produce and require special attention in the course of assembly in order to achieve accurate concentric running of the journals, especially if the frequently used friction [or plain] bearings are replaced by roller bearings. The process of fixing tubes in the boreholes of other parts by expanding the former is known from heat exchanger technology, with both mechanical and hydraulic expanding devices being used.
In German application DE 37 37 601.2 is described a process for producing assembled crankshafts from individually fabricated and subsequently assembled webs and journals in the case of which the journals designed as hollow cylinders are plastically deformed by being expanded and made to contact the respective bores in the webs, with the latter being deformed elastically. This process is suitable for those crankshafts where the crank and bearing journals do not overlap in axial projection. Such crankshafts, even if they are produced in one piece, are subject to loads adversely affecting the strength of the parts, which is the reason why recently so-called short stroke engines have been given preference in the case of which the crank webs are shortened and the crank and bearing journals if necessary increased in diameter to such an extent that the latter partially overlap in axial projection.